Body Mass Index (BMI) is a simple ratio of weight to height used worldwide to screen adults for underweight, healthy weight, overweight and obesity. It's not a diagnosis — it's a first pass. Doctors use it because it's fast, free and correlates well with body-fat levels across populations.
How BMI is calculated
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)². Both formulas produce the same value; the 703 constant just converts pounds/inches into the metric result.
WHO BMI categories
- Under 18.5 — Underweight
- 18.5–24.9 — Normal (healthy) weight
- 25.0–29.9 — Overweight
- 30.0–34.9 — Obese class I
- 35.0–39.9 — Obese class II
- 40.0+ — Obese class III (severe)
Worked example
You are 170 cm tall and weigh 70 kg. Height in metres = 1.70. Height² = 2.89. BMI = 70 ÷ 2.89 = 24.2, which sits in the normal range (18.5–24.9). To reach the top of the range you could weigh up to 72.0 kg.
When BMI is misleading
- Elite athletes and body-builders with high muscle mass often score "overweight".
- Older adults may score "normal" while carrying low bone density and high fat.
- People of Asian descent face metabolic risk at lower BMI thresholds (some guidelines use 23).
- Pregnant and breastfeeding women should not use BMI at all.
Better readings alongside BMI
Pair BMI with waist circumference (over 94 cm men / 80 cm women signals higher risk) and waist-to-height ratio (aim for under 0.5). A DEXA or bioimpedance scan gives your true body-fat percentage.